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5τ rule  In a system which is characterized by a time constant τ the response to a transient process can be determined at 99% level of confidence after the period of 5τ 1τ 2τ 3τ 4τ5τ frequency switching measurement In a system with FM signal and lock-in detection with time constant τ, the modulation frequency should be at least 5/τ or higher t fsw = 50 µs → τ = 10 µs → f mod >500 kHz f mod should not exceed linewidth!!! t fsw = 500 µs → τ = 100 µs → f mod >50 kHz (typically 90 – 100 kHz) time